Material handling machine

ABSTRACT

A material handling machine having a laterally shiftable, elevated carriage on which a vertically shiftable lift head is mounted is provided with controls which operate the machine to automatically perform the multiple functions of picking up material from a receiving bed in response to the accumulation of a predetermined length of material thereon, lifting the material to only the minimum height required to clear a stack of material on which the material is being stacked in response to the height of the stack, carrying the picked up material laterally to a predetermined point above the receiving stack and lowering the material onto the stack or other receiving surface in response to the actuation of a control switch by the carriage when it reaches the predetermined point. The downward stroke of the lift head is automatically stopped in response to the forming of slack or free play in the lift head drive mechanism when the lift head strikes a contact surface on its downward stroke.

[451 Aug. 26, 1975 [57] ABSTRACT A material handling machine having a laterally shiftable, elevated carriage on which a vertically shiftable lift head is mounted is provided with controls which operate the, machine to automatically perform the multiple functions of picking up material from a receiving bed in response to the accumulation of a predetermined length of material thereon, lifting the material to only. the minimum height required to clear a stack of material on which the material is being stacked in response to the height of the stack, carrying the pickedaup material laterally to a predetermined point above the receiving stack and lowering the material onto the stack or other receiving surface in response to the actuation of a control switch by the carriage when" it reaches the predetermined point. The downward "stroke of the lift head is automatically stopped in response to the forming of slack or free play in the lift head drive mechanism when the lift head strikes a contact surface on its downward stroke.

16 Claims, 9 Drawing Figures P.O. Box 453,

214/6 FS; 214/1 ET 1111. B65g 57/04 1 BT, 1 BB, 214/6 P 214/164 214/6 FS 214/1 BT 214/6 0 214/6 FS X MATERIAL HANDLING MACHINE Inventor: Thomas E. Streckert Abbottsford, Wis. 54405 Filed: May 11, 1973 Appl. No.: 359,684

Field of Search...... 214/6 G, 6 FS,

References Cited UNITED STATES PATENTS 7/1964 6/1971 Brown...... 11/1971 Berg1ing.... 10/1972 Bobolts..... 9/1973 Cox et United States Patent Streckert 1| I UEIKiIi I 1 Primary Examiner-Frank E. Werner Attorney, Agent, or Firm-Williamson, Bains & Moore SI. T 414 PATENTEB AUG 2 6 I975 MATERIAL HANDLING MACHINE BRIEF SUMMARY OF THE INVENTION The material handling machine of this invention has been designed and built with a view towards automatically handling material, particularly sheet material, with maximum efficiency in the course of the multiple steps of picking up pieces of material from a supply station, moving laterally with the material and depositing it at a predetermined location at a receiving or stacking station.

These basic objectives have been realized by utilizing a horizontally shiftable carriage on an elevated guide track, mounting a vertically shiftable lift head with material pickup means on the carriage and providing controls to automatically operate the carriage and lift head to carry out the aforesaid material handling steps.

A particularly advantagaeous control feature of the machine resides in the use of a lift head lowering control which is responsive to the generation of slack in the lift head drive mechanism when the lift head strikes a contact surface, to thereby stop the lift head downward stroke immediately to avoid damage to the machine and to the material being handled. In the preferred form of the machine, a chain drive operated by a power cylinder is utilized to raise and lower the lift head, and when the lift head strikes a contact surface, a spring acting on one end of the drive chain shifts the slackened chain to carry a trip device attached to the chain into contact with the actuator of a control switch which then operates to stop the downward movement of the lift head.

As a further beneficial feature of my material handling machine, I provide it with a multiple pickup control function in accordance with which the lift head of the machine is lowered to pick up material from a receiving bed only after a predetermined length of material has accumulated on the bed. This is preferably accomplished by locating a control device, such as a photoelectric cell, adjacent the receiving bed' in front of a material stop a predetermined distance substantially equal to the total length of material desired to be picked up. The control operates in response to the presence of material on the receiving bed at the control location for a predetermined period of time to actuate the lift head on its downward stroke to pick up the material accumulated on the receiving bed betweenthe stop and the control device. I

A height control function is incorporated in the machine and utilizes a sensing device responsive to the height of a stack of material onwhich the machine is placing pieces of material to terminate the upward movement of the lift head with a piece of material at a supply station when the lift head reaches a level above the height of the material stack. This feature eliminates unnecessary elevation of the lift head and initiates the lateral movement of the lift head to the stacking station at a point above the material being stacked when the lift head has risen high enough to clear the stack on which it must deposit the piece of material it has picked up at the supply station. These and other objects and benefits of my invention will become readily apparent as the following description is read in conjunction with the accompanying drawings wherein like reference numerals have been used to designate like elements throughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of the material handling machine of this invention; FIG. 2 is a horizontal section view taken along lines 22 of FIG. 3 and showing the laterally shiftable carriage of the machine;

FIG. 3 is aside elevation view of the machine of FIG. 1 performing a material stacking function;

FIG. 4 is a fragmentary, side elevation view of the machine of FIG I taken from the opposite side of the machine from that shown in FIG. 3 and illustrating the mechanism for raising and lowering the lift head;

FIG. 5 is a fragmentary, left end elevation view of the machine shown in FIG. 1',

FIG. 6 is a control diagram illustrating the various control steps of the machine;

FIG. 7 is a top, plan view of a receiving bed with controls mounted thereon for controlling the total length of accumulated material which the machine picks up at one time; 7

FIG. 8 is a front, elevation view of the receiving bed and controls shown in FIG. 7; and

FIG. 9 is atop, plan view of the lift head for the material handling machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS for this purpose..An upper set of rollers 9 and 10 attach to a horizontal extension 12 of carriage 8 engage the top and rear, side-faces of upper guide track 4; and a bottom roller 14 engages the front face of lower guide track 6.

It will be appreciated that various types of reversible motor means may be utilized to move carriage 8 back and forth on the guide tracks 4 and 6 of boom 1. I have found that a pneumatic cylinder is particularly suitable for this purpose; such a cylinder and its associated piston being designated by reference numerals l6 and 18 in FIGS. 2 and 3( .'The mounting and drive arrangement of cylinder 16 ,with respect to carriage 8 may best be understood by reference to FIGS. 1 through 3 and 5. Pneumatic cylinder 16 is mounted on the vertical web la of I-beam l as shown in FIG. 5, and a bracket 20 attached to piston 18 is secured to an elongated cable 22 guided around .sheaves 24 and 26 rotatably supported on vertical axes mounted within the slotted ends of beam web la in the manner shown in FIG. 1. The ends of cable 22 are secured to a rearward extension 8a of carriage 8 by means of brackets 27 and 28 attached to the opposite sides thereof. The flow of pressurized air rocate piston 18 In opposlte directions IS controlled by a pair of control devices in the form of solenoid valves 5-] and S-2.

as most clearly appears in FIG. 5, I have utilized rollersto the opposite ends of pneumatic cylinder 16 to recip- A lift head generally indicated by reference numeral 30 is mounted on carriage 8 for lateral shifting movement therewith by vertically reciprocal carrier means. I have found it particularly advantageous to utilize a carrier means as shown comprised of a plurality of extensible and retractible lift beams 32, 34 and 36 which slidably support each other, and which are driven by a chain drive arrangement hereinafter described to provide a mechanical lift advantage. The lift apparatus may best be understood by reference to FIGS. 1 and 4.

Upper lift beam 32 is fixed at its bottom end to horizontal plate 8b of carriage 8 and slidably supports beam 34 by means of a slide head 38 on the upper end of lift beam 34 which embraces lift beam 32. A similar slide head 40 is mounted on the bottom end of lift beam 34 and slidably supports and guides bottom lift beam 36. At its lower end, lift beam 36 is secured to cross beam 42 of lift head 30. For the purpose of raising and lowering lift beams 34 and 36, I utilize bidirectional drive means in the form of a pair of drive chains 44 and 46. Chain 44 is fixed at one end to a spring biased rod 48 extending through base plate 8b of carriage 8. The opposite end of chain 44 is attached to slide head 38, with chain 44 being guided around a pair of sprockets 47 and 48 mounted on a bracket 50. Chain 46 is guided around a sprocket 52 attached to slide head 38 and is affixed at one end to base plate 8b of carriage 8. At its opposite end, chain 46 is attached to lift head 30. As may be noted with respect to the showing of chain 46 in FIG. 1, both chains 44 and 46 may be double chains guided around a pair of sprockets. As a source of power for driving chains 44 and 46, I utilize a reversible, doubleacting pneumatic cylinder 52 having a reciprocating piston 54 attached at its upper end to mounting bracket 50 on which sprockets 47 and 48 are mounted. The flow of pressurized air to opposite ends of cylinder 52 to reciprocate piston 54 upwardly and downwardly is controlled by a pair of solenoid valves S-3 and S-4.

For the purpose of precisely controlling the downward stroke of lift head 30 so that it will stop immediately upon contacting a surface of any kind, I utilize-a control switch designated L-7 having a trip arm 56 disposed in proximity to one end of chain 44. The end of chain 44 near switch L-7 carries a trip bar 58 for actuating arm 56 of switch L-7. That same end of chain 44 is attached to a rod 60 which extends through base plate 812 of carriage 8 and is normally biased in a downward direction by a coil spring 62 on its lower end. It will be apparent that the load of lift beam 34 and lift head 30 normally acting downwardly on one end of chain 44 will keep chain 44 in tension, with the result that spring 62 will be compressed in a normally loaded condition by the upward force of segment 44a of chain 44 acting through rod 60. This will cause trip bar 58 to be held at a normal, elevated position out of contact with arm 56 of switch L-7. There is enough free play or slack between drive chains 44, 46 and the lift beams 34 and 36 to which they are attached that when lift head 30 engages a contact surface on its downward stroke to thereby remove the tensionstress normally exerted on chains 44 and 46 by the weight of the lift head and the lift beams, chain 44 will be freely movable through a limited distance by the unloading action of spring 62. Spring 62 bears against the bottom base of carriage base plate 8!), and as it unloads downwardly, it will take up any slight slack generated in chain 44, and thereby urge chain segment 44:! downwardly. As

chain 440 is shifted downwardly, trip bar 48 carried thereon will engage and trip actuating arm 56 of control switch L-7. When this happens, operating rod 58 of switch L-7 will be shifted downwardly to the position shown in FIG. 4 wherein it opens the contacts leading to solenoid valve 84 and thereby shuts off the supply of pressurized air to the upper end of cylinder 52 to stop the downward stroke of piston 54. This will of course immediately terminate the downward movement of chains 44 and 46, and thus immediately stop the downward stroke of lift head 30 upon contact with a surface of any kind. When switch L-7 is actuated in the aforesaidmanner, it simultaneously closes the contacts completing a circuit to solenoid valve 5-3 to ready lift cylinder 52 for the upward stroke of piston 54 and the elevation of lift head 30.

It will be appreciated from the foregoing description of the lift apparatus, that the use of at least one drive chain 44 or 46 in combination with power cylinder 52 provides a mechanical advantage with respect to the vertical movement imparted to lift head 30 by a particular displacement of piston 54 of cylinder 52. With chain44' guided around sprockets 47, 48 attached to piston 54, the segment 44b of chain 44 attached to slide head 38 will be displaced vertically twice the distance through which piston 54 moves in a downward or upward stroke. Thus, lift beam 32 will move twice as far as piston 54; and with a second chain 46 utilized in the manner shown and connected to lift head 30, bottom lift beam 36 attached to lift head 30 will shift vertically four times the distance that piston 54 does on any particular upward or downward stroke. Thus, this particular lifting apparatus does not need an undue amount of space above support boom 1 to support lift head 30 in its position of maximum elevation, and thus does not require an unduly high ceiling for its use. Those skilled in the art will also recognize that other types of drive means other than the multiple chain drive shown may be utilized in combination with a biasing spring to actuate control switch L-7 in response to the generation of slack in the drive means when lift head 30 strikes a contact surface. For example, there is enough slack or free play between gear teeth that this same control principle could be utilized with a rack and pinion type of lift arrangement for lift head 30.

With reference to FIGS. 1 and 4, it may be seen that material pickup means are provided on lift head 30 in the form of a plurality of suction cups 66a, 66b, 66c, and 66d. These cups are attached to mounting blocks 68a, 68b, 68c and 68d for adjustable mounting on. a pair of slide bars 70 and 72. Bars 70 and 72 are provided with longitudinalslots 74, 74a and 76, 76a to accommodate the sliding movement of suction cup blocks 68a 68d thereon. To this end, fasteners extending through mounting blocks 68a 68d and through the slots in bars 70 and 72 are releasably secured thereto by wing nuts 78. Slide bars 70 and 72 are also slidably mounted on cross beam 42 of lift head 30 by means of slide brackets 80 and fasteners 82. By loosening fasteners 82, bars 70 and 72 may be slidably adjusted transversely of lift head 30 on beam 42 to move suction cups 661! 66d in and out transversely of lift head 30. Thus, the combi nation of slidably adjustable mounting blocks 68a 68b and slide bars 70 and 72 permits suction cups 660 66d to be adjusted longitudinally and transversely of lift head 30 to permit picking up varying numbers of pieces of different sizes. Although four suction cups have been means, such as magnetic pickup devices, could be satisfactorily used in place of the suction cups.

As may be noted with respect to both FIGS. I and 4, suction cups are connected through flexible lines 84a, 84b, 84c and 84d with a four-way valve 86 connected in a main vacuum line 88. Vacuum line 88 is connected to a vacuum source, such as the vacuum pump shown inafter with respect to a complete operating cycle of at 90 in FIG. 4. Four-way valve 86 is controlled by a pair of oppositely acting solenoids S- and 8-6 which operate to open and close this valve to supply and cut off the vacuum to suction cups 661: 66d.

In FIG. 1 I have shown a conveyor 92 onto which pieces of material, such as sheets of plywood or panels of any kind are received from a processing machine, and which serves as a supply station at which lift head 30 picks up pieces of material and conveys them to a stacking station or other location for further processing. A photoelectric cell 94 is mounted on one end of slide bar 72 of lift head 30 in the manner shown in FIGS. I and 3 to accomplish a height control function with respect to the elevation of lift head 30, as may best be understood by reference to FIG. 3. In FIG. 3, I have illustrated the material handling machine in the process of performing a stacking operation wherein lift head is picking up pieces of plywood from conveyor 92 and;

depositing them at a stacking station where the wood panels 96 are stacked to a desired height on a pallet 98. The wood panels may obviously be stacked in any number of piles or stacks 99 and 100 comprising a stacking station. In such a material stacking operation, the lift heads of stacking machines have been traditionaly controlled and operated to raise to a predetermined height after picking up a piece of material at a supply station before moving transversely to a stacking station, regardless of the height of the stack of material which must be cleared by the lift head. This necessarily involves a great deal of wasted motion of the lift head and lost time when the stack onwhich a piece of material is to be placed is lower than the predetermined height to which the lift head is always raised. This inefficient type of material handling in a stacking operation is eliminated by the use of photoelectric cell 94 in combination with a light source 102 comprised of a plurality of beam projecting lights 102a. Light source 102 is adjustably mounted on a standard 104 on the opposite side of stacks 99 and 100 from conveyor 92. Beam lights 102a are positioned to send light signals along horizontal paths which intersect the vertical direction of extent of material stacks 99 and 100 accumulating at the stacking station. Photoelectric cell 94 functions as a signal sensing device and is mounted on slide beam the machine. It is sufficient to note at this point that the logic circuits shownin FIG. 6 are set up so that as each step is activated by a control switch, the preceding step is deactivated and the next succeeding step is energized for actuation. It will be seen that control switch L-8 is connected in series with solenoid valve 8-2 in the cir- .cuit of step 3. Thus, when the contacts of switch L-8 are'closed in response to the sensing of a light signal from one of the beam lights 102a by photoelectric cell 94, solenoid 8-3 which controls the upward movement of power piston 54 as shown in FIG. 4, is de-energiz ed and closed to terminate the upward movement of lift head 30, and solenoid valve 5-2 is energized and opened. As may be noted by referenceto FIGS. 2 and 3, solenoid valve S-2 permits the flow of pressurized fluid to the right end of pneumatic cylinder 16 so as to move piston 18 to the left, and thereby pullcable 22 in such a direction as to move carriage 8 to the right, for a stacking right operation. 4

It will be apparent that with one of the stacks 99 or 100 at a relatively high level as indicated by the phantom lies in FIG. 3, the stack of material willblock the transmission of a light signal to photoelectric cell 94 when it is at the level with lift head 30 shown in phantom lines just starting to move upwardly with a piece of material from conveyor 96. Thus, photoelectric cell 94 will not receive a light signal from one of the beam lights 102a until it reaches a level above the height of stacks 99 and 100. When lift head 30 and photoelectric cell 94 reach such a height, control switch L-8 will be actuated in response to the reception of a light signal by photoelectric cell 94, and will function to close solenoid valve S-3 and to open solenoid valve 8-2 to stop the raising of lift head 30, and to initiate the lateral movement of carriage 8 by pneumatic cylinder16 in a direction towards the right to convey the piece of wood panel 96 picked up from conveyor 92 to a position over 72 of lift head 30 so as to be directed towards light source 102 to receive light signals emitted by beam lights 102a. Photoelectric cell 94 operates to close the contacts across a switch designated L-8. Control switch L-8 is connected in the control circuit shown in FIG. 6 in such a way as to stop the upward movement of lift head 30 and to initiate the lateral movement of carriage 8 from a position over conveyer 92 towards the right, as viewed in FIG. 3, to a position over one of the stacks 99 or 100 when its contacts are closed. The control circuit shown in FIG. 6 includes a logic circuit comprised of a number of steps corresponding to operating functions of the machine and is explained completely here one of the stacks 99 or 100. If the stacks 99 and are at a level below lift head 30 when it picks up a piece of material from the top of conveyor 92, then photoelectric cell 94 will immediately receive a light signal and actuate switch L-8 so that lift head 30 will rise only a minimum clearance distance above the top of conveyor 92 and then start moving laterally towards stacks 99 and 100 immediately.

In order to insure that lift head 30 will always rise minimum clearance distance above the top of conveyor 92, or other supply source, I utilize a limit switch L-3 in series with switch L-8 in the circuit of step 3 shown in FIG. 6. Limit switch L-3 is mounted on an upright bar 106 clearlysshown in FIG. l,as well as in FIG. 3 at a predetermined elevation to permit the lifting of head 30 to a minimum clearance .heightabove the top of conveyor 92. Limit switch L-3 is actuated by trip bar l08vcarried on lift beam 34 when beam 34, and. lift head'30tcarried thereby reach the predetermined, min- 1 imum clearance height. Thus, it will be seen that both limit switch L-3 and control switch L-8 under the control of photoelectric cell 94 must be actuated before the lift head elevating function stops, and carriage 8 is shifted laterally to the right.

It should be noted that other types of signal sensin devices. such as an ultrasonic control device may be utilized in place of photoelectric cell 94 to sense the height of material stacks 99 and 100 and carry out the aforesaid .lift height control function.

In FIGS. I and 3 there is shown an elongated bar I09 suspended under elevated Support boom 1 which is utilized to support a plurality of limit switches L-I, L-I l, L-lO and L-2 at the positions shown thereon. These limit switches are utilized in pairs, i.e., L-I, L-ll and L-lO, L-2 to stop the lateral movement of carriage 8 at precise, predetermined positions to permit the alternate stacking or picking up of wood panels at or from multiple stacks or supply sources. For example, limit switches L-l and L-Z are utilized to alternately stop the lateral movement of carriage 8 on a stacking right operation above stacks 99 and 100. Each of these limit switches have actuating arms positioned to be triped by movement of carriage 8 into contact therewith, the actuating arms for two of the switches L-l and L-ll being clearly shown in FIG. I. As is indicated in the control diagram of FIG. 6, limit switches L-2 and L-l0 are connected in parallel with each other, both in series with solenoid valve -4 in step 4 of the logic and control circuit. The logic circuits are arranged so that only one of the switches L-2 or L-lO is in the control circuit to solenoid 8-4 at one time. When one of these limit switches is actuated by carriage 8, it stops the carriage and simultaneously actuates a circuit switching the other limit switch L-l0 into the control circuit for the next stacking operation. Thus, as carriage 8 first moves to the right as viewed in FIG. 3 after picking up a piece of wood paneling 96 from conveyor 92, only switch L-2 is in the control circuit and carriage 8 moves all of the way to the right until it actuates switch L-2. Switch L-2 simultaneously opens the circuit through solenoid valve S-2 and closes the circuit through solenoid valve 5-4. This has the effect of stopping the movement of carriage' 8 towards the right, by shutting off the flow of pressurized fluid to the right end of power cylinder 16, and of permitting pressurized fluid to flow into the upper end of lift head cylinder 52 through solenoid valve 8-4 to initiate the downward movement of lift head 30 over stack 100. The next time that carriage 8 moves to the right after picking up another piece of plywood or wood paneling 96 from conveyor 92, it is stopped by limit switch L-10 over stack 99, switch L-l0 now being in the circuit, and switch L-2 being out of the control circuit. Switch L-l0 performs the same functions as described above with respect to switch L-2 and opening solenoid valve 5-2 in closing solenoid valve 8-4 to stop the movement of carriage 8, and to initiate the downward movement of lift head to deposit the piece of plywood on top of stack 99. Obviously, any number of stacks may be utilized in combination with the corresponding required number of limit switches to alternately stack pieces of material in different stacking locations adjacent to each other.

It will also be appreciated by those skilled in the art, that switches L-l and L-lI can be used in the same manner as described with respect to switches L-IO and L-2 to alternately actuate solenoid valve 8-1 to stop the movement of carriage 8 in a direction towards the left as viewed in FIG. 3 to permit picking up or depositing pieces of plywood or other sheet material from or on opposite sides of conveyor 92. Also, stacks or material handling stations 99 and 100 could serve as alternate sources of supply of pieces of material being fed onto a conveyor, such as conveyor 92 to supply material to a processing machine. With such an operation, limit switches L-2 and L-l0 would function in the same manner as described above to permit lift head 30 to alternately pick up pieces of material from stacks 99 and 100.

In FIGS. 7 and 8 I have illustrated the manner in which additional controls may be utilized in conjunction with my material handling machine to actuate the downward stroke of lift head 30 to pick up a predetermined length of material accumulated on a receiving bed in the form of one article, or a plurality of articles. Reference numeral designates a processing machine from which articles, such as sheets of plywood, are deposited on a receiving bed in the form of a conveyor 92a. Such a conveyor would be, located with respect to pedestal 2 and the material handling machine in essentially the same manner as is conveyor 92 shown in FIG. 1. Conveyor 92a is illustrated as a belt conveyor comprised of a pair of belts 92b and 920. Positioned at one end of conveyor 92a at a predetermined location thereon is a stop means in the form of a stop bar 112 pivotally mounted on an upright support arm 114 by a pivot bracket plate 116. Pivot bracket 1 I6 pivots about point 116a at its point of connection to the upper end of support arm 114. An additional control switch in the form of a limit switch L-6 is mounted on support arm 114 with its actuating arm positioned to be tripped by the rearward displacement of stop bar 112 about pivot point 116:. A spring 118 acting between support arm 114 and bracket plate 116 normally biases plate 116 and stop bar 112 forwardly so that plate 116 will be out of engagement with actuating arm 120 of limit switch L-6. In order that stop bar 112 may be adjustably positioned with respect to receiving bed 92a for the-accumulation of a predetermined length of material, support arm 114 is slidably mounted on a guide rail 122 by a slide bracket 124, as shown in FIG. 3 as well as in FIGS. 7 and 8. A control device which may take various forms, and which is shown for illustrative purposes as a photoelectric cell controller comprised of a light source 1260 and a light sensitive, receiving cell 126/) is positioned at a predetermined location adjacent to receiving bed 92a. The photoelectric cell controller 126a 126i; is located with respect to bed 92a so that it will be a predetermined distance forwardly of stop bar 112 in the direction from which material is received from processing machine 110 such that the distance between stop bar 112 and the photoelectric controller will be substantially equal to a predetermined total length of material to be accumulated on conveyor 92a to be picked up at one time by lift head 30. The light source 126:: for the photoelectric cell is positioned under conveyor 92a between conveyor belts 92b and 920 so that if a sheet of material or other article stops over it, the transmission of the light beam to receiving .cell 126/; will be blocked. This will have the eflect of closing contacts within receiving cell 12612, which is set to have its contacts closed when the light source to it is interrupted. Photoelectrc cell 1261; is connected in the control circuit shown in FIG. 6 in step I of the logic circuit, in series with solenoid control valve S-4 which controls the downward stroke of cylinder piston 54 and of lift head 30. As is illustrated in FIG. 6, a time delay relay 128 is connected in the control circuit in conjunction with the contacts of photoelectric cell 126/2 so as to be energized when the contacts of photoelectric cell 126!) are closed. Time delay relay 128 is set to permit short lengths of material to move past the location of the photoelectric controller, over light source 126a without closing its contacts and completing the circuit through solenoid control valve S-4. Thus, a plurality of short pieces of sheet material can pass onto conveyor 920 without actuating the down stroke of lift head 30. When a single piece of predetermined length, or a plurality of pieces of material equal to the total distance along receiving bed 92a between stop bar 112 and controller 126a l26b have accumulated on the receiving bed, the transmission of light from source 1261: to cell 1261; will be permanently blocked. Thus, the time delay period for which time delay relay 128 is set will be exceeded and it will close its contacts to complete a circuit through solenoid valve S-4. As may be noted with respect to FIG. 4, solenoid controller S-4 directs the flow of pressurized fluid to the upper end of power cylinder 52 to move piston 54 downwardly and to lower lift head 30. Lift head 30 will be lowered by the drive chain arrangement described above until it contacts the material accumulated on conveyor 92a, and will pick up this material and carry it to a stacking station or other processing station as desired. The longitudinal adjustment of support arm 114 on guide rail 122 permits stop bar 112 to be longitudinally adjusted on the receiving bed towards and away from controller 126a 12612 to permit adjusting the distance therebetween for the particular length of material desired to be accumulated from processing machine 110, or from any other source.

Limit switch L-6 shown in FIGS. 1, 7 and 8 is connected in series with the photoelectric cell controller 126:: 12612 in the manner shown in the control circuit diagram of FIG. 6. A manual switch designated SW-l is connected in parallel with the photoelectric cell controller as shown in FIG. 6, and in series with limit switch L-6. By closing switch SW-l, the photoelectric cell controller can be bypassed and the machine will operate without the multiple pickup control function. It will be appreciated that with limit switch L-6 connected in the control circuit in such a manner, it must be actuated by the rearward displacement of stop bar 112 when a piece of material contacts bar 112 before the circuit to lift head control solenoid S-4 can be closed. When switch SW-l is closed, the photoelectric cell controller 126a 1261; will be bypassed, and the tripping of limit switch L-6 when a piece of material is received on conveyor 9211 will actuate solenoid valve 54 and cause lift head 30 to move downwardly to pick up the piece of material.

A complete operating cycle of the material handling machine involving the several functions and control operations of the machine set forth above will now be described. For illustrative purposes, a so-called stack right cycle will be described, wherein the machine is picking up material from the conveyor 92 shown in FIG. 3 and moving to the right and stacking the pieces of material at the stacking station comprised of stacks 99 and 100. As is indicated in FIG. 6, there are six basic steps in the cycle, each step containing a logic section and an amplifier section. The logic section is turned on by a limit switch and the amplifier section is turned on by the logic section, The cycle shown starts with step 1. When the start switch for the machine (not shown) is turned on, step I is activated. The cycle starts with carriage 8 positioned at an elevated level over conveyor 92 at the left end of boom 1 as viewed in FIG. 3. Solenoid 4 will not be actuated to move piston 54 downwardly, and to lower lift head 30 until limit switch L-6 is actuated to indicate that a piece of material has been received on conveyor 92 and is in position to be picked up. If manual switch SW-l is open, and the multiple pickup function controlled by the photoelectric controller 126:1- l26b is operational, then this controller must also be actuated, and the contacts of time delay relay 128 closed in response to the accumulation of a predetermined length of material on the conveyor 92 before solenoid valve S-4 is energized. When this happens, piston 54 of the power cylinder 52 is moved downwardly, and the drive arrangement utilizing drive chains and the lift beams 34 and 36 lowers lift head 30 to pick up a piece of material from conveyor 92. When the suction cups 66a 66b of lift head 30 contact the piece of material, limit switch L-7 is actuated in the manner described above to de-energize solenoid valve 84 and terminate the downward movement of the lift head 30. The closing of switch L-7 also activates or turns on step 2 and completes a circuit through sole-' noid valves S-3 and S-5. Solenoid valve 85 shifts fourway control valve 86, shown most clearly in FIG. 9, to its open position to connect the vacuum pump with each of the suction cups 66a 66d. In this manner, the piece of wood paneling or other sheet material is picked up by lift head 30, and the simultaneous actuation of solenoid valve S-3 shifts piston 54 upwardly to raise lift head 30. Lift head 30 will move upwardly until trip shoe 108 .actuates limit switch L-3 indicating that the lift head has reached the minimum clearance height above the conveyor 92; and, if photoelectric cell 94 senses that the lift head 30 is above the height of the stacking piles 99 and 100, limit switch L-8 will be closed and the circuit through step 3 and solenoid control valve S-2 will be completed. Step 2 will simultaneously be deactivated. The actuation of solenoid valve 5-2 will cause carriage 8 to be shifted to the right by power cylinder 16, and when one of the limit switches L-I0 or L-2 is actuated, step 3 will be turned off and the circuit through step 4 will be completed. Carriage 8 will thus be stopped at a position over one of the stacks 99 or IOOand the actuation of solenoid valve 5-4 in step 4 will cause piston 54 and lift head 30 to move downwardlyto deposit the piece of material on top of one of the stacks. When the piece of wood paneling 96 contacts the stack, limit switch L-7 will again be actuated to stop the-downward movement of lift head 30. This will turn off step 4 and complete the circuit through step -5 to actuate solenoid valves S-6 and S-3. Solenoid valve S-6 shifts four-way control valve 86 to its closed position to shut off the vacuum to the suction cups 660 66b and solenoid valve S-3 supplies pressurized fluid to thebottom of lift cylinder 52 to raise piston 54 and the lift head 30. At the same time, a time delay relay designated TD-l shown in FIG. 6 is energized so that if the lift head is already above the conveyor 92 and limit switch L-3 is actuated lift head 30 will raise another slight distance upwardly before goinginto step 6 and moving laterally to the left. The closing of limit switch- L-3 and the energizing of time delay relay TD-l completes the circuit through step 6, and energizes solenoid controller S1. As may be notedwith reference to FIG. 2, solenoid controller 5-] directs the flow of pressurized fluid into power cylinder 16 at one end thereof so as to shift carriage 8 to the left as viewed in FIGS. 2 and 3. Carriage 8 will move to the left until it contacts one of the limit switches L-] or L-ll which will operate to turn off step 6 and to energize or turn on step 1. At this time, a complete cycle will have been completed and carriage 8 will be in position above conveyor 92 ready to pick up another piece of material, if there are one or more pieces of material on conveyor 92 serving to actuate limit switch L-6 and/or controller 126a 12612 to initiate another cycle.

Limit switch L- shown at the top of lift beam 30 in FIG. 1 is vertically positioned on bar 106 to be actuated by trip shoe 108 to shut off the machine when a pile of material such as that shown at stacks 99 and 100 has reached a'predetermined, maximum height. An additional limit switch could be utilized with its actuating arm under a stack of material being picked up, so that when the last piece of material is removed from the stack, this limit switch will operate to shut off the machine after that piece is conveyed to its processing station.

I anticipate that various changes can be made in the structure and operating mechanism and controls of the material handling machine described herein without departing from the spirit and scope of my invention as defined by the following claims.

I claim:

1. A material handling machine comprising:

a lift head having material pickup means thereon;

a carrier means attached to said lift head, said carrier means being vertically shiftable upwardly and downwardly to raise and lower said lift head;

bidirectional drive means in driving association with said carrier means, there being sufficient free play between said drive means and said carrier means that when said lift head engages a contact surface on its downward stroke and thereby removes stress normally exerted on said drive means by said carrier means, said drive means will be freely movable through a limited distance, said drive means being operable in a first direction to raise said carrier means and said lift head and in the opposite direction to lower said carrier means and said lift head;

spring means engaging said drive means and maintained in a normally loaded condition by the stress normally exerted on said drive means by said carrier means;

a control switch operable in one position to stop the movement of said drive means in said opposite direction to limit the downward movement of said carrier means and lift head; and

a trip device for said control switch carried on said drive means, the unloading action of said spring when said pickup means engage a contact surface on the downward stroke of said lift head serving to move said drive means through said limited distance to thereby carry said trip device into a predetermined position wherein it actuates said switch to said one position wherein it stops the movement of said drive means in said opposite direction-and terminates the downward movement of said carrier means and lift head.

2. A material handling machine as defined in claim 1 wherein:

said bidirectional drive means comprises at least one chain guided around a sprocket with one fixed end secured in biasing, coacting relation with said spring means and having its opposite end attached to said carrier means, whereby the load of said carrier means and said lift head on said one end of said chain normally maintains said chain in tension against the biasing pressure of said spring means,

and the tension on said chain is thus removed when said pickup means engages a contact surface on the downward stroke of said lift head, thereby producing slack in said chain and permitting said spring to move said fixed end of said chain and said trip device therewith through said limited distance to actuate said control switch.

3. A material handling machine as defined in claim 2 wherein:

a power cylinder is attached to said chain sprocket and is operable to raise and lower said sprocket and said chain therewith, the flow of pressurizing fluid to said power cylinder in a direction to lower said sprocket, chain and carrier means being controlled by said control switch.

4. A material handling machine as defined in claim 1 wherein:

said material pickup means comprises a plurality of vacuum cups connected to a vacuum source, said cups being adjustably mounted on said lift head for shifting movement longitudinally and transversely of said lift head, whereby said vacuum cups may be adjusted on said lift head to pick up varying numbers of pieces of different sizes.

5. A material handling machine as defined in claim 1, and further including a receiving bed on which material is received and accumulated to be picked up by said machine;

displaceable material stop means at a predetermined location on said receiving bed for stopping pieces of material conveyed onto and along said bed at a predetermined point thereon; and

a control switch mounted adjacent to said stop means and operable in response to the displacement of said stop means to actuate said drive means for movement in said opposite direction to lower said lift head.

6. Material handling apparatus comprising:

a horizontally shiftable carriage on an elevated, horizontal guide track;

reversible motor means drivingly associated with said carriage and operable to move said carriage back and forth on said track; 7

a lift head mounted on said carriage for vertical movement upwardly and downwardly, said lift head being horizontally shiftable with said carriage;

material pickup means on said lift head;

means for raising said lift head;

a motor means control device operative to initiate movement of said motor means in one direction to shift said carriage in a predetermined, horizontal direction on said track;

a supply station at which pieces of material to be picked up by said material handling machine are deposited;

a stacking station at which material picked up from said supply station by said machine is stacked;

a signal emitter positioned to send a signal along a horizontal path which intersects the vertical direction of extent of a stack of material accumulating at said stacking station and on which said material handling machine is placing pieces of material picked up from said supply station;

1 a signal sensing device mounted on said lift head and operative in response to a signal received from said signal emitter to deactivate said lift head raising means and to actuate said motor means control device to initiate movement of said carriage in said predetermined direction, whereby, with said lift head moving upwardly with a piece of material picked up from said supply station, the stack of material accumulating at said stacking station will interfere with the transmission and receptionof said signal by said emitter and sensing device and said sensing device will not deactivate said lift head raising means and actuate said motor means control device until said lift head and said sensing .device thereon have been elevated to a level above the top of the stack of material accumulating at said stacking station. I I v 7. Material handling apparatus as defined in claim 6 wherein:

8. Material handling apparatus as defined in claim 7 wherein:

a height control limit switch is interposed in said circuit with said photoelectric cell and said motor means control device and is operative in response to the elevation of said lift head to a'predetermined minimum clearance heightto close a set ofv contacts in saidcircuit to said motor control device. 9. Material handling apparatus as defined in claim 6 wherein:

said apparatus includes means for lowering said head;

said stacking station is comprised of first and second material receiving stations; and

first and second limit switches spaced apart along the length of said track in substantially vertical alignment with said first and second material receiving stations respectively, each of said limit switches being operative in a control circuit in response to contact by said carriage to actuate said lift head lowering means, said limit switches being connected in a logic circuit of said control circuit in such a manner that only one of said switches is in said control circuit at a time, whereby said limit switches alternately actuate said lift head lowering means in response to contact by said carriage to cause the alternate lowering of said lift head at first one and then the other of said material receiving stations to stack pieces of material at said stations.

10. Material handling apparatus comprising:

a material handling machine having a lift head movable upwardly and downwardly;

material pickup means on said lift head;

means for lowering said lift head to pick up material;

a receiving bed on which material is received and accumulated;

material stop means at a predetermined location on said receiving bed for stopping pieces of material conveyed onto and along said bed at a predetermined point thereon;

a control device positioned at a location adjacent to said receiving bed which is a predetermined distance forwardly of said stop means in the direction from which material is received, such that the distance between said stop means and said control device is substantially equal to a predetermined total length of material to be accumulated on said bed,

said control device including a time delay relay and being operative in response to the presence of material on said bed at said location for a predetermined period of time to actuate said means for loweringsaid lift head to pick up the material accumulated on said'bed, said time delay relay being set to permit short lengths of material to move past said control device location onto said bed towards said stop means without activating said control device, whereby said control device will not actuate said means for. lowering said lift head until a plurality of pieces of material or a single piece of material having a length equal to said predetermined total length have accumulated on said receiving bed. 11. Material handling apparatus as defined in claim l0 wherein:

said control device is a light-sensitive photoelectric cell controller having at least a portion thereof so positioned under said receiving bed at said location that the presence of material on said bed at said location will block the transmission of light to said controller and thereby initiate the lift head control function of said photoelectric cell controller.

12. Material handling apparatus as defined in claim l0 wherein:

said material stop means is longitudinallyadjustable on said receiving bed towards and away from said control device to permit adjusting the distance therebetween for the particular length of material desired to be accumulated on said receiving bed.

13. Material handling apparatus as defined in claim 10 wherein:

said pickup means on said lift head comprises a plurality of material holding devices adjustably mounted on said lift head for movement longitudinally thereof in a direction extending lengthwise of said receiving bed, whereby said pickup means may be spaced and positioned on said lift head to pick up different sizes and accumulated lengths of material from said receiving bed.

14. Material handling apparatus as defined in claim 10 wherein:

said means for lowering said lift head comprises vertically shiftable carrier means attached to said lift head and bidirectional drive means drivingly associated therewith, there being sufficient free play between said drive means and said carrier means that when said pickup means engages material on said receiving bed on the downward stroke of said lift head to thereby remove stress normally exerted on said drive means by said carrier means, said drive means will be freely movable through a limited distance;

spring means engaging said drive means and maintained in a normally loaded condition by the stress normally exerted on said drive means by said carrier means; i

a control switch operable in one position to stop the movement of said drive means in a direction lowering said carrier means and lift head, said switch having an actuator disposed in proximity to said drive means; and

a trip device for said switch actuator carried on said drive means, the unloading action of said spring when said pickup means strikes material on said receiving bed serving to move said drive means through said limited distance to thereby carry said trip device into engagement with said switch actuator and move said switch to said one position wherein it stops the movement of said drive means in said lift head lowering direction.

15. A material handling machine comprising:

a horizontally shiftable carriage on an elevated horizontal guide track;

reversible motor means drivingly associated with said carriage and operable to move said carriage back and forth on said track;

a motor means control device operable to initiate movement of said motor means in one direction to shift said carriage in a predetermined, horizontal direction on said track;

a lift head mounted on said carriage for vertical movement upwardly and downwardly, and horizontally shiftable with said carriage;

material pickup means on said lift head;

means for lowering said lift head;

a work station at a first location along said guide track;

first and second material handling stations adjacent to each other at a location along said guide track between said work station and one end of said guide track at which the same material stacking or picking-up functions are carried out in proximity to said machine;

first and second limit switches spaced closely together along the length of said track in substantially vertical alignment with said first and second material handling stations rcspectively, each of said limit switches being operative in a control circuit to actuate said lift head lowering means in response to contact by said carriage as it moves in the same predetermined direction towards said first and second material handling stations, said limit switches being connected in a logic circuit of said control circuit in such a manner that only one of said switches is in said control circuit at a time, whereby said limit switches selectively and intermittently actuate said lift head lowering means in response to contact by said carriage as it moves in said predetermined direction to cause the selective and intermittent lowering of said lift head at said material handling stations to carry out the same material handling function at each of said stations.

16. A material handling machine as defined in claim 1 wherein: I

said control switch has an actuator disposed in proximity to said drive means, and said trip device is carried into engagement with said switch actuator and thereby moves said switch to said one position as said drive means moves through said limited distance. 

1. A material handling machine comprising: a lift head having material pickup means thereon; a carrier means attached to said lift head, said carrier means being vertically shiftable upwardly and downwardly to raise and lower said lift head; bidirectional drive means in driving association with said carrier means, there being sufficient free play between said drive means and said carrier means that when said lift head engages a contact surface on its downward stroke and thereby removes stress normally exerted on said drive means by said carrier means, said drive means will be freely movable through a limited distance, said drive means being operable in a first direction to raise said carrier means and said lift head and in the opposite direction to lower said carrier means and said lift head; spring means engaging said drive means and maintained in a normally loaded condition by the stress normally exerted on said drive means by said carrier means; a control switch operable in one position to stop the movement of said drive means in said opposite direction to limit the downward movement of said carrier means and lift head; and a trip device for said control switch carried on said drive means, the unloading action of said spring when said pickup means engage a contact surface on the downward stroke of said lift head serving to move said drive means through said limited distance to thereby carry said trip device into a predetermined position wherein it actuates said switch to said one position wherein it stops the movement of said drive means in said opposite direction and terminates the downward movement of said carrier means and lift head.
 2. A material handling machine as defined in claim 1 wherein: said bidirectional drive means comprises at least one chain guided around a sprocket with one fixed end secured in biasing, coacting relation with said spring means and having its opposite end attached to said carrier means, whereby the load of said carrier means and said lift head on said one end of said chain normally maintains said chain in tension against the biasing pressure of said spring means, and the tension on said chain is thus removed when said pickup means engages a contact surface on the downward stroke of said lift head, thereby producing slack in said chain and permitting said spring to move said fixed end of said chain and said trip device therewith through said limited distance to actuate said control switch.
 3. A material handling machine as defined in claim 2 wherein: a power cylinder is attached to said chain sprocket and is operable to raise and lower said sprocket and said chain therewith, the flow of pressurizing fluid to said power cylinder in a direction to lower said sprocket, chain and carrier means being controlled by said control switch.
 4. A material handling machine as defined in claim 1 wherein: said material pickup means comprises a plurality of vacuum cups connected to a vacuum source, said cups being adjustably mounted on said lift head for shifting movement longitudinally and transversely of said lift head, whereby said vacuum cups may be adjusted on said lift head to pick up varying numbers of pieces of different sizes.
 5. A material handling machine as defined in claim 1, and further including a receiving bed on which material is received and accumulated to be picked up by said machine; displaceable material stop means at a predetermined location on said receiving bed for stopping pieces of material conveyed onto and along said bed at a predetermined point thereon; and a control switch mounted adjacent to said stop means and operable in response to the displacement of said stop means to actuate said drive means for movement in said opposite direction to lower said lift head.
 6. Material handling apparatus comprising: a horizontally shiftable carriage on an elevated, horizontal guide track; reversible motor means drivingly associated with said carriage and operable to move said carriage back and forth on said track; a lift head mounted on said carriage for vertical movement upwardly and downwardly, said lift head being horizontally shiftable with said carriage; material pickup means on said lift head; means for raising said lift head; a motor means control device operative to initiate movement of said motor means in one direction to shift said carriage in a predetermined, horizontal direction on said track; a supply station at which pieces of material to be picked up by said material handling machine are deposited; a stacking station at which material picked up from said supply station by said machine is stacked; a signal emitter positioned to send a signal along a horizontal path which intersects the vertical direction of extent of a stack of material accumulating at said stacking station and on which said material handling machine is placing pieces of material picked up from said supply station; a signal sensing device mounted on said lift head and operative in response to a signal received from said signal emitter to deactivate said lift head raising means and to actUate said motor means control device to initiate movement of said carriage in said predetermined direction, whereby, with said lift head moving upwardly with a piece of material picked up from said supply station, the stack of material accumulating at said stacking station will interfere with the transmission and reception of said signal by said emitter and sensing device and said sensing device will not deactivate said lift head raising means and actuate said motor means control device until said lift head and said sensing device thereon have been elevated to a level above the top of the stack of material accumulating at said stacking station.
 7. Material handling apparatus as defined in claim 6 wherein: said motor means control device is electrically operable; said signal emitter is a light source; and said sensing device is a photoelectric cell operative to open and close a circuit to said control device.
 8. Material handling apparatus as defined in claim 7 wherein: a height control limit switch is interposed in said circuit with said photoelectric cell and said motor means control device and is operative in response to the elevation of said lift head to a predetermined minimum clearance height to close a set of contacts in said circuit to said motor control device.
 9. Material handling apparatus as defined in claim 6 wherein: said apparatus includes means for lowering said head; said stacking station is comprised of first and second material receiving stations; and first and second limit switches spaced apart along the length of said track in substantially vertical alignment with said first and second material receiving stations respectively, each of said limit switches being operative in a control circuit in response to contact by said carriage to actuate said lift head lowering means, said limit switches being connected in a logic circuit of said control circuit in such a manner that only one of said switches is in said control circuit at a time, whereby said limit switches alternately actuate said lift head lowering means in response to contact by said carriage to cause the alternate lowering of said lift head at first one and then the other of said material receiving stations to stack pieces of material at said stations.
 10. Material handling apparatus comprising: a material handling machine having a lift head movable upwardly and downwardly; material pickup means on said lift head; means for lowering said lift head to pick up material; a receiving bed on which material is received and accumulated; material stop means at a predetermined location on said receiving bed for stopping pieces of material conveyed onto and along said bed at a predetermined point thereon; a control device positioned at a location adjacent to said receiving bed which is a predetermined distance forwardly of said stop means in the direction from which material is received, such that the distance between said stop means and said control device is substantially equal to a predetermined total length of material to be accumulated on said bed, said control device including a time delay relay and being operative in response to the presence of material on said bed at said location for a predetermined period of time to actuate said means for lowering said lift head to pick up the material accumulated on said bed, said time delay relay being set to permit short lengths of material to move past said control device location onto said bed towards said stop means without activating said control device, whereby said control device will not actuate said means for lowering said lift head until a plurality of pieces of material or a single piece of material having a length equal to said predetermined total length have accumulated on said receiving bed.
 11. Material handling apparatus as defined in claim 10 wherein: said control device is a light-sensitive photoelectric cell controller having at least a portion thereof so positIoned under said receiving bed at said location that the presence of material on said bed at said location will block the transmission of light to said controller and thereby initiate the lift head control function of said photoelectric cell controller.
 12. Material handling apparatus as defined in claim 10 wherein: said material stop means is longitudinally adjustable on said receiving bed towards and away from said control device to permit adjusting the distance therebetween for the particular length of material desired to be accumulated on said receiving bed.
 13. Material handling apparatus as defined in claim 10 wherein: said pickup means on said lift head comprises a plurality of material holding devices adjustably mounted on said lift head for movement longitudinally thereof in a direction extending lengthwise of said receiving bed, whereby said pickup means may be spaced and positioned on said lift head to pick up different sizes and accumulated lengths of material from said receiving bed.
 14. Material handling apparatus as defined in claim 10 wherein: said means for lowering said lift head comprises vertically shiftable carrier means attached to said lift head and bidirectional drive means drivingly associated therewith, there being sufficient free play between said drive means and said carrier means that when said pickup means engages material on said receiving bed on the downward stroke of said lift head to thereby remove stress normally exerted on said drive means by said carrier means, said drive means will be freely movable through a limited distance; spring means engaging said drive means and maintained in a normally loaded condition by the stress normally exerted on said drive means by said carrier means; a control switch operable in one position to stop the movement of said drive means in a direction lowering said carrier means and lift head, said switch having an actuator disposed in proximity to said drive means; and a trip device for said switch actuator carried on said drive means, the unloading action of said spring when said pickup means strikes material on said receiving bed serving to move said drive means through said limited distance to thereby carry said trip device into engagement with said switch actuator and move said switch to said one position wherein it stops the movement of said drive means in said lift head lowering direction.
 15. A material handling machine comprising: a horizontally shiftable carriage on an elevated horizontal guide track; reversible motor means drivingly associated with said carriage and operable to move said carriage back and forth on said track; a motor means control device operable to initiate movement of said motor means in one direction to shift said carriage in a predetermined, horizontal direction on said track; a lift head mounted on said carriage for vertical movement upwardly and downwardly, and horizontally shiftable with said carriage; material pickup means on said lift head; means for lowering said lift head; a work station at a first location along said guide track; first and second material handling stations adjacent to each other at a location along said guide track between said work station and one end of said guide track at which the same material stacking or picking-up functions are carried out in proximity to said machine; first and second limit switches spaced closely together along the length of said track in substantially vertical alignment with said first and second material handling stations respectively, each of said limit switches being operative in a control circuit to actuate said lift head lowering means in response to contact by said carriage as it moves in the same predetermined direction towards said first and second material handling stations, said limit switches being connected in a logic circuit of said control circuit in such a manner that only one of said switches is in said control circuit At a time, whereby said limit switches selectively and intermittently actuate said lift head lowering means in response to contact by said carriage as it moves in said predetermined direction to cause the selective and intermittent lowering of said lift head at said material handling stations to carry out the same material handling function at each of said stations.
 16. A material handling machine as defined in claim 1 wherein: said control switch has an actuator disposed in proximity to said drive means, and said trip device is carried into engagement with said switch actuator and thereby moves said switch to said one position as said drive means moves through said limited distance. 